A method for culturing embryonic rat liver in a chemically defined medium was developed and by the use of this method an in vitro model for liver carcinogenesis was obtained. 11 Day embryonic rat liver was cultured in a chemically defined medium for up to 16 weeks. Liver cells maintained their histological and ultrastructural appearance as well as their enzymatic activity (glycogen synthetase) during this period. N-2 fluorenyl-acetamide (2-FAA) was successfully used for transformation of the organ culture within 7 weeks. The early effects of 2-FAA on DNA synthesis and the ultrastructural alteration of transformed liver cells are currently being studied.